1. Field of the Invention
The invention is concerned with a magnetic material having high permeability in the high frequency range, including a plurality of magnetic metal layers alternating with electrically insulating layers, together with means for electrically short-circuiting the magnetic metal layers locally between the layers.
2. Description of the Prior Art
As is known from the prior art, ferrites have been widely used as core materials for magnetic transducer heads. Because of the improved characteristics of present-day magnetic recording media, and particularly the requirement for a high coercive force (Hc), there is a recent trend toward the use of metallic materials such as "Sendust", "Permalloy", "Alperm" and amorphous magnetic alloys such as Co--Nb--Zr and Co--Ta--Zr. As the magnetic recording techniques advance, the signal frequency range to be used is raised. For example, there is a demand for magnetic materials which have high permeability in the ultra-high frequency range, for example, in excess of 10 MHz and particularly from several tens MHz to 100 MHz.
As is well known, the specific resistance of magnetic metal materials such as the amorphous magnetic metals or "Sendust" is as low as about 100 .mu. ohm.cm. When these magnetic metal materials are used as a core material, the permeability is lowered due to eddy current losses in the high frequency signal range. In order to prevent the occurrence of eddy currents and prevent the lowering of permeability in the high frequency range, it is common to use a magnetic core having a laminated structure. This type of core is formed from the magnetic metal material as mentioned above in a thickness such that the eddy current loss is negligible, superimposing another layer on the magnetic metal layer and consisting of an electrically insulative layer, and repeating the above procedure to form a laminated core having a predetermined thickness.
However, when such a magnetic core of laminated construction is used with the application of an ultra-high frequency signal in the high MHz range, a high frequency eddy current loss takes place with the result that the expected degree of high permeability cannot be achieved. We believe that this is caused by the fact that the two adjoining magnetic metal layers and the insulative layer between them constitute a capacitor and the impedance of the capacitor decreases with an increase in frequency. Consequently, in the above-indicated ultra-high frequency range, particularly in the range of several tens MHz to 100 MHz or higher, the eddy current passes through the capacitor. Thus, materials which ordinarily have high permeability, high saturation magnetic flux density, and similar desirable properties, provide the serious problem of lowering of permeability due to eddy current loss at ultra-high frequencies. A multi-layer laminated arrangement is not the answer because the incorporation of the insulator between two magnetic metal layers provides a capacitor through which eddy current flow can occur at such high frequencies.